Impacts of fauna on an upland grassland soil as determined by micromorphological analysis

It is widely recognised that soil fauna are distinguished by their abundance and diversity. However, there is a surprising lack of knowledge on the precise functional roles played by many animals within soils. This is the basis to the UK NERC Thematic Programme on Biological Diversity and Ecosystem Function in Soil in an upland grassland ecosystem. In this programme 19 team projects have the overall objective of assessing such biological diversity and ecosystem function. This paper reports preliminary results from a project designed to investigate the interactions between the activity of fauna and soil structure. The approach is based on investigating the nature and distribution of excremental pedofeatures using soil micromorphology. The experimental site, at an elevation of 320 m, is situated on the Macaulay Land Use Research Institute's Sourhope Research Station in the Bowmont valley to the south of Kelso in the Scottish Borders. The vegetation at the site is an acid upland grassland, dominated by Agrostis capillaris, developed on a brown forest soil (Sourhope Series) which has been cultivated in the past. For the upper soil horizons, eight types of excremental pedofeatures are identified according to the size, shape and composition. In both the organic horizon (H) and the underlying mineral horizon (Ah), the bulk of the soil volume consists of excremental pedofeatures derived from enchytraeids, earthworms with excrement either in vermiform or mammilated forms and surface feeding animals such as beetles. Enchytraeid excrement increases with depth from 13% in the LF to 29% in the Ah horizon. Excrement from oribatid mites is only present to a limited extent (4%) in the LF horizon. Earthworm excrement is present in all horizons. Within the lower part of the H horizon, some profiles have a narrow (1-1.5 cm) dark grey organic layer dominated by phytoliths, and also distinguished by having fewer excremental features. The key finding is the extent to which excrement from a fairly small range of soil fauna is dominant in the upper organic and organo-mineral horizons. Overall, the results demonstrate a close relationship between soil horizons and faunal activity as expressed in excremental pedofeatures. (C) 2002 Elsevier Science B.V. All rights reserved.